They converted without hello(?).

[Marcus Darkstar]

In my campaign with KH Chersonesos converted to my faction without saying anything, no fancy historical document.

Should I expect this to happen again?

And by the way, do I need to destroy "Pastoralism" ?

Its due to the cultural and Govenor's building in the settlement. There are various Indepdent Hellenic polis across the mediterrean like Sinope who can convert to KH's faction if they are taken over by a faction and then rebell.

Pastoralism is for nomads :P you dont have to destroy it but youll continue to retain a cultural negetive influence in that settlement with that.